Table of Contents
Since the borates which supply commerce with most of the raw materials for conversion into borax as it is used in the arts now come from old lake-beds or inland-sea deposits, their chemical relations and development are quite like those of saline deposits generally. While a general sequence of salts in the precipitations from complex saline waters has been commonly regarded as established, it is now known that this succession is not everywhere invariably the same. Neither is the sequence in inverse order of solubility, as it was long thought to be.
The experiments on evaporating large quantities of sea-water carried on many years ago by the celebrated Italian scientist, Usiglio, are well known. The results obtained by this chemist have been widely accepted; but more recent tests prove that they are not of so wide application as was at first supposed. Careful comparisons show that the artificial processes do not correspond exactly to the natural ones. This fact recently led the German chemists, Van’t Hoff, Meyerhoffer, Hindrichsen, and Weigat, to conduct exhaustive researches on the salt-formations in nature. Very interesting results were obtained, which throw a flood of light upon the subject, and offer satisfactory explanations to many hitherto little understood phenomena.
Among the important factors which Usiglio, and others who have been especially interested in similar experimentation, did not take into consideration were:
- the composition of the saline waters;
- the solubility of the compounds present;
- the time allowed for concentration;
- the temperature at which saturation for a given salt took place; and
- pressure under which crystallization began.
Since the recent chemical results have such a direct bearing upon the saline deposits under consideration, they may be briefly summed up here.
In the great salt-deposits of Stassfurt, Germany, which were chiefly investigated, it was found that in the succession of strata four very distinct zones were recognizable. These, beginning at the bottom and named after the principal salt found in them, were the anhydrite zone, the polyhalite zone, the kieserite zone, and the carnallite zone. In all of these zones rock-salt is found. There are also other salts present which are regarded as of secondary formation.
The desiccated inland-sea deposits of the Great Basin region of western America have not been as yet investigated in detail to determine the full variety of salts and their relationships. However, sufficient is known in the case of the borate-deposits of the Death Valley district to state something regarding the peculiar conditions existing at the time at which the salts belonging to the first or lowest zone were precipitated. This zone is the one containing, besides anhydrite, the borates, gypsum, calcite, and some other salts in which lime is an important constituent.
Composition of Saline Waters
Were it merely oceanic waters with which we had to deal the chemistry of natural salines would be very simple. By not taking into account the calcium salts the composition would be identical the world over. The composition of the waters of bitter-lakes is very much more complex and varied. Many new conditions are introduced. Enclosed bodies of water, especially those of the very dry regions of the earth, receive compounds in solution from the surrounding elevations that vary greatly in every case, and according to the composition of the rocks, or geologic terranes. In every known instance some one salt greatly predominates.
Instead of the various salts being precipitated in inverse order of solubility, it appears that in a given solution the component which is greatly in excess is the one that is most likely to reach the point of saturation first, and hence will be the first to crystallize out. As Van’t Hoff has recently clearly shown, concentration will continue until the water reaches the point of saturation for a second salt, when that also will commence to be precipitated. If for the moment we can neglect the other salts, in order to give the problem its simplest form, it is from this point onward that the water remains with the composition unchanged. The water gradually evaporates and the salts continue to fall until complete desiccation has taken place.
Solubility of Components
There is a widespread opinion among scientists that the salts which crystallize out of saline waters in the arid regions of the globe are merely in solution, and that merely the proper point of concentration is required to be reached in order to precipitate a given salt. Such, it has been already intimated, is not really the case.
Recent observation has conclusively shown that in the desiccation of some saline waters certain salts which naturally would be expected to be found do not appear at all. In other cases compounds entirely unexpected are actually deposited. Under one set of physical conditions the waters of bitter-lakes as they evaporate may throw down a certain series of salts, while under slightly different conditions the same saline waters may deposit an entirely distinct series of compounds.
The first-mentioned results are rather unduly emphasized on account of their being the outcome of laboratory-experimentation also. Here the physical conditions are always very nearly uniform, and the methods of chemical procedure fixed. In nature there is no such uniformity of conditions as is found in the laboratory. In consequence there are many departures from the artificially-conducted tests. Solubility is also a function of temperature, and varies in degree very greatly, as all laboratory-work shows.
Time-Element in Water-Concentrations
In nature the time-factor in the determination of precipitates in solution is probably very much more important than is commonly assumed. In the chemical laboratory time is of necessity practically eliminated in all experimentation, and as a consequence very erroneous conclusions are often drawn regarding the chemical processes at work in the earth’s crust and the results attained.
The unexpected chemical reactions in nature are as noteworthy in the desiccation of saline waters as they are among the rock-magmas in the process of solidification. Among the last mentioned granite alone may be cited out of the many known examples. It is shown that an acidic magma, owing to the presence of aqueous vapor, the high pressures under which alone granite can form, and the long time that must manifestly pass, may cool down considerably below the temperature required to crystallize out certain minerals under ordinary dry-fusion conditions. Thus quartz, which should be formed quite early in the normal sequence, can be the last to crystallize, solidifying the whole mass into solid rock. This principle was long ago formulated by Scheerer, who later advocated it at greater length and in greater detail. It was subsequently confirmed experimentally by Elie de Beaumont, Daubree and others, as well as by some more recent investigators.
In the case of similar retardations in crystallization of salts in saline waters under much simpler conditions than those existing among molten materials, recent inquiry has clearly indicated that such phenomena occur very much more frequently than was ever surmised. Length of time, however, is not the only determining factor in these cases.
In the formation of natural salts in desiccating lake-waters the time-factor must be regarded as of prime importance. To it must be ascribed the presence in the sequence of saline deposits of certain salts which never appear in the laboratory- trials. Concerning the saline deposits of the old inland seas of the Great Basin region, this time-factor explains much that previously was very obscure.
Effect of Temperature
The general influence of temperature in effecting the crystallizations in saline solutions need not be dwelt upon at length here. Effects of the high temperatures are now well known. Effects of slight changes of a few degrees, within the limits of the ordinary temperatures as they are known in saline waters of the arid regions, have not been so well understood or considered.
At normal temperatures saline waters of the desert basins may deposit a certain number of salts and in a certain sequence. Under conditions of 20° or 30° increase waters of identical composition in the process of desiccation may give rise to some entirely new minerals. At the same time, at the higher temperature, some of the salts which commonly appear at lower degrees of heat do not form at all. Within certain limits the salts derived from evaporation of the waters of saline or bitter- lakes may be regarded as indices of the temperatures of the waters at the time the deposits took place. Hence, it is possible to use deposits of this kind as factors in the determination of geologic climate.
Temperature of saline waters has also a very important bearing upon the paragenesis of many of the minerals which are commonly associated in old lake-beds or deposits of inland seas. The gathering of winter and summer sodas in some of the alkaline ponds of Wyoming and elsewhere forms a good illustration. Just what part temperature has played in the formation of the borate-deposits of California has not yet been definitely determined, but it is thought to be highly influential.
Effect of Pressure
The effect of pressure in the formation of saline materials in saline lake-waters can hardly be so great as it is in the cases of many other geologic deposits. Variations in pressure must be quite negligible, because the bodies of water of this kind are comparatively shallow when the salts begin to form. In laboratory-experimentation pressure is usually eliminated altogether.
Boraciferous Formations
The Tertiary boraciferous formations of southern California are the most remarkable and most extensive in the world. They were formed under conditions of an arid climate in a great shallow arm of the Pacific ocean that had been cut off by the upheaval of the mountain ranges along the coast. The inland sea was long in drying up, and perhaps had frequent connection with the ocean, as is shown by the enormous thickness of the terranes carrying the borates. The disappearance of the water may have been more rapid than the thickness of the deposits suggests at first thought, for the reason that as an accompaniment of the evaporation of the waters in an excessively dry climate there may have been a filling-up of the basin by the prodigious quantities of wind-borne dust derived from the neighboring deserts. It is not to be inferred that, since the Tertiary clays and sands are between 5,000 and 8,000 ft. thick, the waters in the beginning were at least of the same depth, but rather that the arm of the ocean and afterwards the inland sea was always very shallow, and that as the area was filling up the waters continued to rest on the surface, rising with the rise of the bottom. This postulates a gradual sinking of the foundations of the region, and the truth of this is indicated by the general tectonics of the region.
The entire field of the Tertiary clays in southern California is capable of great results from systematic prospecting and exploration for commercial salines other than calcium borate. The inferences to be drawn from the modern conceptions of the deposition of salines are that with proper inquiry a large series of natural salts may be discovered. The calcium borate-beds are easily passed over unnoticed unless special care be taken to look for them. Other borates are found even more valuable than the colemanite. Extensive rock-salt deposits are already known, as are those of purest gypsum, anhydrite, and calcite. In some of the bitter-lakes immense bodies of soda and magnesia of the kind known mineralogically as bloedite, are among the most wonderful deposits recently found. In one small lakelet scarcely a mile across it is estimated that more than 1,000,000 tons of this mineral is readily available.